Many vessels in animals transport fluids from one bodily location to another. Frequently, fluid flows in a unidirectional manner along the length of the vessel. Varying fluid pressures over time, however, can introduce a reverse flow direction in the vessel. In some vessels, such as mammalian veins, natural valves are positioned along the length of the vessel and act as one-way check valves that open to permit the flow of fluid in the desired direction and close to prevent fluid flow in a reverse direction, i.e., retrograde flow. The valves can change from an open position to a closed position in response to a variety of circumstances, including changes in the cross-sectional shape of the vessel and the fluid pressure within the vessel.
While natural valves may function for an extended time, some may lose effectiveness, which can lead to physical manifestations and pathology. For example, venous valves are susceptible to becoming insufficient due to one or more of a variety of factors. Over time, the vessel wall may stretch, affecting the ability of the valve leaflets to close. Furthermore, the leaflets may become damaged, such as by formation of thrombus and scar tissue, which may also affect the ability of the valve leaflets to close. Once valves are damaged, venous valve insufficiency may be present, and can lead to discomfort and possibly ulcers in the legs and ankles.
Current treatments for venous valve insufficiency include the use of compression stockings that are placed around the leg of a patient in an effort to force the vessel walls radially inward to restore valve function. Surgical techniques are also employed in which valves can be bypassed, eliminated, or replaced with autologous sections of veins with competent valves.
Minimally invasive techniques and instruments for placement of intraluminal medical devices have developed over recent years. A wide variety of treatment devices that utilize minimally invasive technology has been developed and includes stents, stent grafts, occlusion devices, infusion catheters and the like.
Recently, prosthetic valves that are implantable by minimally invasive techniques have been developed. Frequently, a graft member is attached to a support frame and provides a valve function to the device. For example, the graft member can be in the form of a leaflet that is attached to a support frame and movable between first and second positions. In a first position, the valve is open and allows fluid flow to proceed through a vessel in a first direction, and in a second position the valve is closed to prevent fluid flow in a second, opposite direction. Examples of this type of prosthetic valve are described in commonly owned U.S. Pat. No. 6,508,833 to Pavcnik for a MULTIPLE-SIDED INTRALUMINAL MEDICAL DEVICE, U.S. patent application Publication No. 2001/0039450 to Pavcnik for an IMPLANTABLE VASCULAR DEVICE, and U.S. patent application Ser. No. 10/642,372, filed on Aug. 15, 2003, all of which are hereby incorporated by reference in their entirety. In other examples of prosthetic valves, a tube that terminates in leaflets is attached to one or more support frames to form a valve. The leaflets open to permit fluid flow in a first direction in response to fluid pressure on one side of the leaflets, and close to prevent fluid flow in a second, opposite direction in response to fluid pressure on opposite sides of the leaflets. An example of this configuration is provided in U.S. Pat. No. 6,494,909 to Greenhalgh for AN ENDOVASCULAR VALVE, which is hereby incorporated by reference in its entirety.
Some materials used in prosthetic valves may be capable of adhering to or becoming incorporated into the wall of a vessel in which the valve is implanted. For example, some graft materials may adhere to a vessel wall if thrombus forms on or near the material. Also, some natural materials, such as bioremodellable materials, may become incorporated into the vessel will due to cellular in-growth, bioremodelling, or other processes.
Adherence and/or incorporation of a portion of a prosthetic valve may be desirable in some circumstances. For example, adherence of a portion of a graft material of a prosthetic valve to a vessel wall may facilitate anchoring of the valve within the vessel. However, it may be desirable to prevent adherence and/or incorporation in some circumstances. For example, if a portion of a prosthetic valve requires movement to retain function, such as a valve leaflet or a portion of a leaflet, adherence and/or incorporation of that portion may affect performance of the prosthetic valve. Furthermore, mechanical interaction between a movable portion, such as a valve leaflet or portion of a leaflet, and another structural component, such as a portion of a support frame, may also be undesirable.